VTOL M-wing configuration

ABSTRACT

A vertical landing and take-off aircraft VTOL transitions from a vertical takeoff state to a cruise state where the vertical takeoff state uses propellers to generate lift and the cruise state uses wings to generate lift. The aircraft has an M-wing configuration with propellers located on the wingtip nacelles, wing booms, and tail boom. The wing boom and/or the tail boom can include boom control effectors. Hinged control surfaces on the wings, tail boom, and tail tilt during takeoff and landing to yaw the vehicle. The boom control effectors, cruise propellers, stacked propellers, and control surfaces can have different positions during different modes of operation in order to control aircraft movement and mitigate noise generated by the aircraft.

TECHNICAL FIELD

The described subject matter generally relates to the field of aerialtransportation and, more particularly, to a vehicle for vertical takeoffand landing that can serve multiple purposes, including thetransportation of passengers and cargo.

BACKGROUND

Some existing vehicles in the emerging vertical takeoff and landing(VTOL) aircraft ecosystem rely on separate non-articulating rotors toprovide vertical lift and forward thrust. However, this approach resultsin extra motor weight and aircraft drag since vertical lift rotors areineffective during forward flight. Other existing aircrafts use adistributed set of tilting propulsors that rotate in the direction offlight to provide both vertical lift and forward thrust. While thisapproach reduces motor weight and aircraft drag, the articulating motorand propulsors result in increased design complexity with six to twelvetilting rotors required to provide the necessary lift and thrust.

SUMMARY

In various embodiments, the above and other problems are addressed by aVTOL aircraft that transitions from a vertical takeoff and landing stateprimarily using stacked propellers for lift to a cruise primarily usingone or more wings for lift. In one embodiment, the aircraft has anM-wing configuration with propellers located on wingtip nacelles, wingbooms, and a tail boom. The wing boom and/or the tail boom can includeboom control effectors. Each propeller may be powered by a separateelectric motor. Hinged control surfaces on the wings, tail boom, andtail may tilt during takeoff and landing to yaw the vehicle.

During vertical ascent of the aircraft, rotating wingtip propellers onthe nacelles are pitched upward at a 90-degree angle and stacked liftpropellers are deployed from the wing and tail booms to provide lift.The hinged control surfaces tilt to control rotation about the verticalaxis during takeoff. As the aircraft transitions to a cruiseconfiguration, the nacelles rotate downward to a zero-degree position,allowing the wingtip propellers to provide forward thrust. Controlsurfaces return to a neutral position with the wings, tail boom, andtail, and the stacked lift propellers stop rotating and retract intocavities in the wing booms and tail boom to reduce drag during forwardflight.

During transition to a descent configuration, the stacked propellers areredeployed from the wing booms and the tail boom and begin to rotatealong the wings and tail to generate the lift required for descent. Thenacelles rotate back upward to a 90-degree position and provide boththrust and lift during the transition. The hinged control surfaces onthe wings are pitched downward to avoid the propeller wake, and thehinged surfaces on the tail boom and tail tilt for yaw control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an M-wing configuration of a VTOL aircraft, inaccordance with one or more embodiments.

FIG. 2A is a side view of a stacked propeller, in accordance with one ormore embodiments.

FIG. 2B is a top view of a stacked propeller, in accordance with one ormore embodiments.

FIG. 3 illustrates various configurations of a stacked propeller, inaccordance with several embodiments.

FIG. 4A illustrates a configuration of a stacked propeller during afirst mode of operation, in accordance with one or more embodiments.

FIG. 4B illustrates a configuration of a stacked propeller during asecond mode of operation, in accordance with one or more embodiments.

FIG. 4C illustrates a configuration of a stacked propeller during athird mode of operation, in accordance with one or more embodiments.

FIG. 4D illustrates a configuration of a stacked propeller during afourth mode of operation, in accordance with one or more embodiments.

FIG. 4E illustrates a side view of an aircraft with boom controleffectors, in accordance with one or more embodiments.

FIG. 5 illustrates a climb configuration of a VTOL aircraft, inaccordance with the embodiment of FIG. 1 .

FIG. 6 illustrates an early egress transition configuration of a VTOLaircraft, in accordance with the embodiment of FIG. 1 .

FIG. 7A illustrates a late egress transition configuration of a VTOLaircraft, in accordance with the embodiment of FIG. 1 .

FIG. 7B illustrates a top view of a propeller configuration associatedwith one or more modes of operation, in an accordance with theembodiment of FIG. 7A.

FIG. 7C illustrates a top view of a propeller configuration associatedwith one or more modes of operation, in an accordance with theembodiment of FIG. 7A.

FIG. 8 illustrates a cruise configuration of a VTOL aircraft, inaccordance with the embodiment of FIG. 1 .

FIG. 9 illustrates an early ingress transition configuration of a VTOLaircraft, in accordance with the embodiment of FIG. 1 .

FIG. 10 illustrates a late ingress transition configuration of a VTOLaircraft, in accordance with the embodiment of FIG. 1 .

FIG. 11 illustrates a descent configuration of a VTOL aircraft, inaccordance with the embodiment of FIG. 1 .

DETAILED DESCRIPTION

The figures and the following description describe certain embodimentsby way of illustration only. One skilled in the art will readilyrecognize from the following description that alternative embodiments ofthe structures and methods illustrated herein may be employed withoutdeparting from the principles described herein. Reference will now bemade to several embodiments, examples of which are illustrated in theaccompanying figures. It is noted that wherever practicable similar orlike reference numbers may be used in the figures and may indicatesimilar or like functionality.

1.1 Aircraft Overview

FIG. 1 is an illustration of a vertical takeoff and landing (VTOL)aircraft 100, according to one or more embodiments. The illustrated VTOLaircraft 100 is a transitional aircraft that transitions from a verticaltakeoff state to a cruise state where the vertical takeoff state usespropellers to generate lift and the cruise state uses wings to generatelift. The aircraft 100 is used for transporting passengers and cargo.The aircraft 100 is configured to move with respect to three axes. InFIG. 1 , a roll axis is collinear with the x-axis and a pitch axis iscollinear with the y-axis. A yaw axis is collinear with the z-axis,which is perpendicular to the x-axis and the y-axis (e.g., the z-axisextends from the page). The origin of the coordinate system is fixed tothe center of gravity of the aircraft 100 during one or more modes ofoperation.

The aircraft 100 includes an aerodynamic center and a center of thrust.The aerodynamic center is a point of an aircraft where the aerodynamicmoment is constant. The aerodynamic moment is produced as a result offorces exerted on the aircraft 100 by the surrounding gas (e.g., air).The center of thrust is the point along the aircraft 100 where thrust isapplied. The aircraft 100 includes components strategically designed andlocated so that the aerodynamic center, center of thrust and/or centerof gravity can be approximately aligned. (e.g., separated by a distanceof no more than five feet (1.524 meters)) during various modes ofoperation. The components of the aircraft 100 are arranged such that theaircraft 100 is balanced during vertical and forward flight. Forexample, components such as the control surfaces (e.g., tail controlsurface, boom control effectors), propellers, and M-wing shape functioncooperatively to balance the aircraft 100 during different modes ofoperation.

The aircraft 100 includes an M-wing configured to the body of a fuselage135 and a tail region extending from the rear of the fuselage 135. Inthe embodiment of FIG. 1 , the aircraft 100 includes a port portion anda starboard portion. The wing is arranged in an M-configuration suchthat the port portion and the starboard portion of the wing each has twoangled segments that merge at an inflection point. A first segmentextends outwardly from the fuselage 135 to an inflection point and asecond segment extends outwardly from the inflection point. The firstsegment and the second segment are joined at a non-zero angle at theinflection point. In various embodiments, the angle ranges from 5-25degrees. In other embodiments, other angles may be used.

The leading edge where the angled segments merge (e.g., the inflectionpoint) is the forward most point along each portion of the M-wing. Theleading edge is the part of the wing that first contacts the air duringforward flight. In one embodiment, the inflection point where the angledcomponents merge coincides with the midpoint of each portion (e.g., portportion, starboard portion) of the wing. In one embodiment, the portportion and the starboard portion of the wing can be individualcomponents, each having a wide v-shape. In the embodiment of FIG. 1 ,the wing is a continuous M-configuration, but in alternative embodimentsthe wing includes two separate v-wings (e.g., starboard, port) that areattached to the fuselage 135.

The shape of the M-wing is selected to reduce the surface area creatingdrag during take-off and landing configurations while providingsufficient lift during forward flight. In one embodiment, the wing spanis approximately 30 to 40 feet and the distance from the tip of astarboard cruise propeller 110 to a port cruise propeller 110 (describedin greater detail below) is approximately 40 to 50 feet. The wingsurface area is approximately 110 to 120 square feet. Alternatively, thewing can have any suitable dimensions for providing lift to theaircraft.

In one embodiment, the M-wing includes wing booms 120 where the leadingedge of each wing boom 120 is located at the approximate midpoint ofeach portion of the wing (e.g., the inflection point where the angledsegments of each portion of the wing merge). The wing booms 120 can beattached to the wing at the leading edge and may protrude 1 to 3 feetfrom the leading edge. In one embodiment, the center of mass of the wingbooms 120 is on or ahead of the neutral axis of the wings. The wingbooms 120 can include additional elements, such as batteries, to alignand/or balance the center of gravity of the aircraft 100 during a modeof operation.

In one embodiment, a stacked propeller (e.g., starboard stackedpropeller 115 a, port stacked propeller 115 b) can be attached to a wingboom 120. A stacked propeller (e.g., starboard stacked propeller 115 a,port stacked propeller 115 b) can be located behind the wing in order toprovide lift and stability to the aircraft 100. Locating a stackedpropeller behind the wing allows for improved circulation over the wingand the stacked propeller. As a result, the stacked propeller canprovide a significant contribution to lift during vertical takeoff andlanding. The location of the stacked propeller also allows for alignmentof the aerodynamic center, the center of thrust, and the center ofgravity of the aircraft during different modes of operation.

The aircraft 100 includes a tail region attached to the rear end of thefuselage 135. The tail region can include a tail boom 145 and a tail. Inone embodiment, the aircraft includes a T-tail configured to providestability to the aircraft 100. The T-tail is shaped and located in aposition to provide lift to the aircraft in nominal operation. As such,the T-tail can be referred to as a lifting tail. The T-tail includes atail plane 155 mounted perpendicularly to the top of a fin 448. The fin448 is shown in a profile view of the aircraft 100 in FIG. 4E and caninclude a rudder 457 that rotates to control yaw motion of the aircraft100. The tail plane 155 attached to the top of the fin 448 can includeone or more tail control surfaces 160 located at the rear of the tailplane 155. In one embodiment, the T-tail is configured to position theaerodynamic center over a specified passenger seat (e.g., a rearpassenger seat) so that it is coincident (or approximately coincident)with the center of gravity during vertical flight. The T-tail can alsocontribute to adjusting the aerodynamic center towards the nose of thefuselage 135 (e.g., slightly ahead of the wing) during a cruiseconfiguration.

The T-tail is approximately 4 to 6 feet tall from the base of the fin448 to the top of the tail plane 155 and the tail plane 155 isapproximately 10 to 20 feet wide. The T-tail can be tall enough so thatthe angle of the tail control surface 160 can be varied when one or morepropellers attached to a tail boom induces a negative airflow angle ofattack during a transition configuration (e.g., egress and ingressdescribed in greater detail below). Varying the tail control surface mayreduce any negative impacts of the airflow generated by the propellerson the T-tail during transition. In one embodiment, a navigation lightis located on the rear of the tail to alert other aircrafts of theposition and heading of the aircraft 100. Propellers (e.g., frontstacked propeller 140 a, rear stacked propeller 140 b) can be attachedto the tail boom 145. Alternatively, one or more propellers can belocated at any point along the tail region. Similarly to the stackedwing propellers (e.g., starboard stacked propeller 115 a, port stackedpropeller 115 b), a tail propeller(s) can be located strategically alongthe tail to contribute to alignment of the aerodynamic center, thecenter of thrust, and the center of gravity.

The aircraft 100 relies on propellers for vertical takeoff and landingas described below in relation to FIG. 5 and FIG. 11 . The aircraft 100includes stacked propellers (a starboard stacked propeller 115 a, a portstacked propeller 115 b, a front stacked propeller 140 a, a rear stackedpropeller 140 b) and single rotor propellers (cruise propellers 110) inorder to maximize lift. The propellers may be oriented along the span(e.g, laterally) of the aircraft 100 to prevent interference ofpropeller flows during transition and to minimize power required totransition from a vertical configuration to a cruise configuration. Theposition of the propellers may prevent turbulent wake flow (e.g.,turbulent air flow produced by a propeller) ingestion betweenpropellers. The propellers can be located so that the airflow of onepropeller does not negatively interfere with the airflow of anotherpropeller. The arrangement of the propellers may also allow for a moreelliptically shaped lift and downwash airflow distribution duringtransition configurations to achieve lower induced drag, power, andnoise. In one embodiment, the aircraft 100 has approximately 331 squarefeet of propeller area such that, an aircraft 100 with a mass ofapproximately 4500 pounds has a disc loading is approximately 13.6pounds per square foot. The disc loading is the average pressure changeacross an actuator disc, more specifically across a rotor or propeller.In other embodiments in which the propeller area is approximately 391square feet (e.g., if the diameter of the cruise propellers 110 andstacked wing propellers is approximately 10 feet), the disc loading isreduced to 11.5. Power usage may be decreased when the disc loading isreduced, thus efficiency of an aircraft can be increased by reducing thedisc loading. The combination and configuration of the propellers of theaircraft 100 yields a disc loading that allows the aircraft 100 togenerate enough lift to transport a large load using a reasonable amountof power without generating excessive noise.

1.2 Aircraft Fuselage

Shown in FIG. 1 , the fuselage 135 is located at the center of thewingspan and includes a passenger compartment configured to accommodatepassengers, cargo, and/or a pilot. The fuselage 135 is approximately 35to 45 feet long, approximately 4 to 8 feet wide, and approximately 5 to12 feet tall. In alternative embodiments, the fuselage 135 can have anysuitable dimensions for transporting passengers and/or cargo.

The passenger compartment may include one or more seats for passengers.In one embodiment, the passenger compartment includes seating for up tofour passengers. Seating may be arranged in two parallel rows of twoseats such that the one row of passengers faces the tail of the aircraft100 while the other row of passengers faces the nose (e.g., forwardregion of the fuselage 135) of the aircraft 100. In one embodiment, thepassenger seating can be tiered such that one row of seats is elevatedabove the other row of seats to maximize space and provide a place forpassengers to rest their feet. Alternatively, the seating may bearranged in a single row with two sets of two seats, each of the seatsin the set facing opposite directions such that the passengers in thefirst and third seats face the tail of the aircraft 100 while passengersin the second and fourth seats face the nose of the aircraft 100. Inother configurations, all four seats face the nose or tail of theaircraft 100. The arrangement of passenger seats may have alternateconfigurations in order to distribute the passenger weight in a specificmanner such that the aircraft 100 is balanced during a mode ofoperation. In other embodiments, the fuselage 135 can include a largeror smaller number of seats.

The fuselage 135 can also include a view screen in the passengercompartment for providing information about the flight. For example, theview screen can include information such as estimated arrival time,altitude, speed, information about origin and destination locations,and/or communications from the pilot. The forward region (e.g., regionclosest to the nose of the aircraft 100) of the fuselage 135 includes acockpit with a control panel and seating for a pilot. In one embodiment,the front of the cockpit is located ahead of the horizontal plane of thecruise propellers 110 such that the blades of the cruise propellers 110are not in line with the pilot.

In some embodiments, a battery pack is located below the passengercompartment in the fuselage 135. The battery pack is separated from thebottom surface of the fuselage 135 to facilitate ventilation of thebattery pack. The bottom surface of the fuselage 135 can also include abattery door to allow for removal of the battery pack. In alternativeembodiments, the batteries can be placed above the fuselage 135 andintegral to the wing. The fuselage 135 can include a charging port onthe nose where the aircraft 100 may be attached to a charging station torestore electrical power stored in the batteries that power the aircraft100. Fixed or retractable landing gear may also be attached to thebottom of the fuselage 135 to facilitate landing of the aircraft 100 andallow the aircraft 100 to move short distances on the ground.Alternatively, the aircraft 100 may have landing skis protruding fromthe bottom of the fuselage 135 and include attachment points for wheels.

1.3 Control Surfaces

In the embodiment of FIG. 1 , the aircraft 100 includes wing controlsurfaces 130 that span the trailing edge of the wing. The trailing edgeis the edge opposite to the leading edge of the wing. In one embodiment,each wing portion has three wing control surfaces 130 along the rear ofthe wing: a first wing control surface approximately 5 to 7 feet longbetween the fuselage 135 and a wing boom 120, and a second and thirdwing control surface each approximately 3 to 5 feet long between thewing boom 120 and a wingtip nacelle 112. The wing control surfaces 130can be deployed at different angles during aircraft operation toincrease the lift generated by the wing and to control the pitch of theaircraft 100. The wing control surfaces 130 are hinged such that theycan rotate about a hinging axis that is parallel to the wing. Forexample, the wing control surfaces 130 are in a neutral position duringa parked configuration and are rotated approximately 40 degrees below aplane parallel to the x-y plane to facilitate takeoff. The modes ofoperation of the wing control surfaces 130 are described in more detailbelow in relation to FIGS. 5-11 .

The aircraft 100 can also include control surfaces in other locationsalong the aircraft such as the tail control surface 160 (describedabove) and the rudder 457 (shown in FIG. 4E). The control surfaces onthe tail (e.g., tail control surface 160, rudder 457) can adjust theaerodynamic center of the aircraft 100 such that the aircraft 100 isdynamically stable in different modes of operation. For example, thetail control surface 160 is neutral (i.e., tilted to a zero-degreeangle) during a cruise configuration, and the tail control surface 160tilts approximately 5 to 10 degrees during descent. The rudder 457 isneutral (i.e., tilted to a zero-degree angle) during a transition to acruise configuration, and the rudder 457 tilts approximately 5 to 10degrees during descent to yaw the aircraft 100 into the correctorientation for landing. The rudder 457 can operate in addition to orinstead of boom control effectors, described below, for yaw control. Themodes of operation of the tail control surface 160 and the rudder 457are described in greater detail below in relation to FIGS. 5-11 .

In some configurations, the aircraft 100 can include control surfaces onthe bottom of each of the wing booms 120 and the tail boom 145 that tiltto yaw the aircraft 100. The control surfaces can deflect propeller flowto create control forces resulting in yaw and direct sideslipcapabilities. For example, while the control surfaces are neutral (i.e.,at zero degrees) during a cruise configuration, they rotate slightly(e.g., at approximately five to ten degrees) during descent to yaw theaircraft 100 into the correct orientation. In one embodiment, thecontrol surfaces on the bottom of each of the wing booms 120 and thetail boom 145 are boom control effectors, described in greater detailbelow.

1.4 Cruise Propellers

In one embodiment, an aircraft 100 includes one or more cruisepropellers 110 shown in FIG. 1 . The cruise propellers 110 provide liftto the aircraft 100 during takeoff and landing and forward thrust to theaircraft 100 during a cruise configuration. Shown in FIG. 1 , the cruisepropellers 110 are mounted on nacelles 112 perpendicular to the fuselage135. In one embodiment, the nacelles 112 have a non-circular crosssection to reduce the effect of aerodynamic forces on the aircraft 100.Each nacelle 112 rotates about an axis parallel to the y-axis duringdifferent modes of operation. As discussed in more detail below inrelation to FIGS. 5-11 , during vertical takeoff and landing thenacelles 112 are perpendicular to the fuselage 135 such that the bladesof the cruise propellers 110 rotate in a plane parallel to the x-yplane, facilitating vertical movement of the aircraft 100. As theaircraft 100 enters an egress configuration (i.e., when the aircraft 100is approaching a cruising altitude), the nacelles 112 and cruisepropellers 110 rotate downward (e.g, towards the nose of the fuselage135 about an axis parallel to the y-axis) until the nacelles areparallel to the fuselage 135, facilitating forward thrust of theaircraft 100. As the aircraft 100 enters an ingress configuration (i.e.,when the aircraft 100 begins to descend), the nacelles 112 and cruisepropellers 110 rotate upward (about an axis parallel to the y-axistowards the positive z-direction) until the blades of the cruisepropellers 110 are level in a plane parallel to the x-y plane, wherethey remain during descent and landing of the aircraft 100. In oneembodiment, the cruise propellers 110 can be counter-rotating. Forexample, the port cruise propeller 110 rotates in a clockwise directionand the starboard cruise propeller 110 rotates in a counterclockwisedirection during a mode of operation.

In one embodiment, each of the cruise propellers 110 has five blades,although they may have fewer or more blades in other embodiments. Theblades of the cruise propellers 110 narrow from the root of the blade tothe tip. The cruise propellers 110 may have a fixed pitch (e.g., thecruise propellers 110 are held at a fixed angle of attack).Alternatively, the pitch is variable such that the blades of the cruisepropellers 110 can be partially rotated to control the blade pitch. Thecruise propellers 110 can be driven by separate electric motors. Eachcruise propeller 110 is approximately 8 to 10 feet in diameter and isattached at a 90-degree angle to a nacelle 112 (e.g., the nacelles 112are parallel with the z-axis) at a free end of each portion (e.g.,starboard portion, port portion) of the wing. Alternatively, the cruisepropellers 110 can have any suitable dimensions.

1.5 Stacked Propellers

An aircraft may include one or more stacked propellers. The propellerscan be located on the front, back, port and/or starboard region of theaircraft. In the embodiment of FIG. 1 , the aircraft 100 includes astarboard stacked propeller 115 a and a port stacked propeller 115 bwhere the starboard stacked propeller 115 a and the port stackedpropeller 115 b can be attached to a wing or a wing boom 120 of theaircraft. The embodiment of FIG. 1 also includes stacked tail propellers(e.g., front stacked propeller 140 a, rear stacked propeller 140 b) thatcan be attached to the tail boom 145. Alternatively, a stacked propellercan be located in any other position on the aircraft 100.

A stacked propeller (e.g., starboard stacked propeller 115 a, portstacked propeller 115 b, front stacked propeller 140 a, rear stackedpropeller 140 b) functions to provide lift and thrust to an aircraftduring takeoff and landing. FIGS. 2A and 2B illustrate a side view and atop view of a stacked propeller, according to an embodiment. The stackedpropeller includes a first propeller 260 and a second propeller 262. Thefirst propeller 260 and the second propeller 262 each include two blades269 coupled to a blade hub 268. The blades 269 of the first propeller260 and the second propeller 262 co-rotate about an axis of rotation264. The first propeller 260 and the second propeller 262 can have avariable pitch.

The first propeller 260 can be coupled (e.g., mechanically,electrically) to a first motor 280 and the second propeller 262 can becoupled to a second motor 282 to enable independent control of eachpropeller. The first motor 280 or the second motor 282 can control boththe first propeller 260 and the second propeller 262 in someembodiments. For instance, if the first motor 280 fails (e.g., batterydies), the second motor 282 can control the rotation of the firstpropeller 260 and the second propeller 262. A stacked propeller can alsoinclude a clutch which allows the first propeller 260 and the secondpropeller 262 to lock together to ensure an appropriate azimuth angle266 during a mode of operation. A clutch allows for a stacked propellerto provide thrust from both the first propeller 260 and the secondpropeller 262, even in a case where one of the motors (e.g, first motor280) fails and the other motor (e.g., second motor 282) controls therotation of the first propeller 260 and the second propeller 262. Insome embodiments, a stacked propeller can include a single motor and acontroller with a clutch used to control the azimuth angle 266 that isused in a mode of operation, and in other embodiments a stackedpropeller can include two motors with independent controllers and aclutch used in a case when one of the motors fails. The first motor 280and the second motor 282 can also control the precise azimuth angle 266,shown in FIG. 2B, of the first propeller 260 relative to the secondpropeller 262, when the blades are stationary or in motion. The azimuthangle 266 depends on the mode of operation of the aircraft, described ingreater detail below.

The co-rotating propellers (e.g. first propeller 260, second propeller262) may be synchronized such that they rotate at the same speed toreduce the noise generated by the aircraft 100. The azimuth angle 266 isconstant when the first propeller 260 and second propeller 262 arerotating at the same speed (e.g., during steady flight). The azimuthangle 266 can depend on the shape of the blade 269 and/or the mode ofoperation. For instance, a specified shape, such as the shape shown inFIG. 2B, can have an offset angle of 5-15 degrees during different modesof operation.

The speed of the propellers may be adjusted based on the amount ofthrust required to provide vertical ascent and descent and the amount ofnoise allowable in the geographic area in which the aircraft 100 istraveling. For example, the pilot might lower the speed of the aircraft100, causing the aircraft 100 to climb more slowly, in areas in which alower level of noise is desirable (e.g., residential areas). In oneembodiment, the maximum speed of a free end of each of the blades 269 is450 feet per second. This may keep the noise produced by the aircraft100 below an acceptable threshold. In other embodiments, other maximumspeeds may be acceptable (e.g., depending on the level of noiseconsidered acceptable for the aircraft and/or aircraft environment,depending on the shape and size of the blades 269, etc.).

In one embodiment, a stacked propeller can be encapsulated in a duct265. The duct 265 can surround the blades 269 and a rotor mast 270 toaugment the flow over the first propeller 260 and/or the secondpropeller 262. The duct 265 can function to increase the thrustgenerated by a stacked propeller and/or adjust the pressure differenceabove and below the co-rotating propellers. The first propeller 260 andthe second propeller 262 can be recessed within the duct 265, shown inFIG. 2A. In alternative embodiments, the first propeller 260 can beprotruding from or flush with the duct 265 while the second propeller262 is recessed within the duct 265. Similarly, the rotor mast 270 canbe recessed within or protruding from the duct 265. In the embodiment ofFIG. 2A, the duct 265 is a cylindrical body with a diameter slightlylarger than the diameter of the first propeller 260 and the secondpropeller 262.

Co-rotating propellers may provide an advantage to single rotorpropellers because they can produce less noise. Noise produced bypropellers varies as an exponent of the tip speed of a propeller, thus,in order to reduce noise produced by a single rotor propeller, theaircraft speed is also reduced. A stacked propeller design also allowsfor flexibility of angles between the propellers which can be variedduring different stages of flight functioning to increase the efficiencyof the system. The speed and phase angle can be adjusted for eachpropeller on a stacked propeller, allowing for a more flexible andadaptable system. The stacked propellers can be stored during modes ofoperation where they are not necessary in order to reduce drag andimprove efficiency.

The configuration of a stacked propeller can vary depending on theembodiment and requirements of the aircraft system and/or operationmode. In one embodiment, each co-rotating propeller (e.g., the firstpropeller 260, the second propeller 262) has the same blade shape andprofile while in other embodiments, the first propeller 260 and thesecond propeller 262 have different dimensions and an offset phase ofrotation. For example, the first propeller 260 and the second propeller262 may have different camber and twist such that, when the propellersare azimuthally separated, a stacked propeller (e.g., starboard stackedpropeller 115 a, port stacked propeller 115 b, front stacked propeller140 a, rear stacked propeller 140 b) is able to achieve optimal camberbetween the two surfaces. For example, in one embodiment, the diameterof the second propeller 262 is approximately 95% of the diameter of thefirst propeller 260.

In relation to material composition, a stacked propeller (e.g.,starboard stacked propeller 115 a, port stacked propeller 115 b, frontstacked propeller 140 a, rear stacked propeller 140 b) can be made fromof a single material or can be a composite material able to providesuitable physical properties for providing lift to the aircraft. Thefirst propeller 260 and the second propeller 262 can be made from thesame material or different materials. For example, the first propeller260 and the second propeller 262 can be made from aluminum, or the firstpropeller 260 can be made from steel and the second propeller 262 can bemade from titanium. The blade hub 268 can be made from the same ordifferent material than the first propeller 260 and the second propeller262. Alternatively, the components of the system (e.g., the firstpropeller 260, the second propeller 262, the blade hub 268) can be madefrom a metal, polymer, composite, or any combination of materials. Thestacked propeller may also be exposed to extreme environmentalconditions, such as wind, rain, hail, and/or extremely high or lowtemperatures. Thus, the material of the stacked propeller can becompatible with a variety of external conditions.

In relation to mechanical properties, the material of the firstpropeller 260 and the second propeller 262 can have a compressivestrength, a shear strength, a tensile strength, a strength in bending,an elastic modulus, a hardness, a derivative of the above mechanicalproperties and/or other properties that enable the propeller to providevertical lift to the aircraft. The first propeller 260 and the secondpropeller 262 may experience extreme forces during operation includingthrust bending, centrifugal and aerodynamic twisting, torque bending andvibrations. The material of the first propeller 260 and the secondpropeller 262 can have a strength and rigidity that allows thepropellers to retain their shape under forces exerted on the propellersduring various modes of operation. In one embodiment, the firstpropeller 260 and/or the second propeller 262 are composed of a rigidcomposite. Additionally, the edges or tips of the blades 269 can belined with a metal to increase strength and rigidity.

In one embodiment or during a certain mode of operation, the firstpropeller 260 and the second propeller 262 may co-rotate in a counterclockwise direction. In a different mode of operation, the firstpropeller 260 and the second propeller 262 can co-rotate in a clockwisedirection. In the embodiment of FIG. 1 , the stacked propellers (e.g.,the starboard stacked propeller 115 a, the port stacked propeller 115 b)along the aircraft can rotate in opposite directions based on the modeof operation. For example, the starboard stacked propeller 115 a canrotate in a clockwise direction and the port stacked propeller 115 b canrotate in a counter clockwise direction. The stacked propellers (e.g.,the front stacked propeller 140 a, the rear stacked propeller 140 b) canalso rotate in the same or opposite directions. For example, the frontstacked propeller 140 a and the rear stacked propeller 140 b can bothrotate in a clockwise direction during a mode of operation. Therotational direction of a stacked propeller may depend on the mode ofoperation. According to the embodiment in FIG. 1 , the stackedpropellers (e.g., starboard stacked propeller 115 a, port stackedpropeller 115 b, front stacked propeller 140 a, rear stacked propeller140 b) have a diameter of approximately 6 to 10 feet. Alternatively, thestacked propellers can have any suitable dimensions. The stacked tailpropellers (e.g. front stacked propeller 140 a, rear stacked propeller140 b) can operate in addition to or instead of the starboard stackedpropeller 115 a and stacked port propeller 115 b. The above descriptionis not exclusive of the possible combinations of directions of rotationfor each stacked propeller. The examples are used for illustrationpurposes.

FIG. 3 illustrates a first embodiment (top left), a second embodiment(top right), a third embodiment (bottom left), and a fourth embodiment(bottom right), of a stacked propeller. A first embodiment (top left)shows a top view of a stacked propeller including a first propeller 360a and a second propeller 362 a with angular blades 369 a. The firstpropeller 360 a and the second propeller 362 a each includes two blades369 a. The width of the blades 369 a is narrower at the blade hub 368 athan at the free end of the blades 369 a. A second embodiment (topright) of FIG. 3 includes a first propeller 360 b with three blades 369b and a second propeller 362 b with three blades 369 b. The blades 369 bare wider at the blade hub 368 b than at the free ends of the blades 369b. The free ends of the blades 369 b are round. A third embodiment(bottom left) of FIG. 3 shows a schematic including a first propeller360 b and a second propeller 362 b each including two blades 369 bcoupled to a blade hub 368 c. The blades 369 b of the propellers arewider at the blade hub 368 c than at the free end. The diameter of thesecond propeller 362 c is smaller than the diameter of the firstpropeller 360 c. A fourth embodiment (bottom right) of FIG. 3 includes apropeller with a first propeller 360 d and a second propeller 362 d,each including two blades 369 d coupled to a blade hub 368 d. The blades369 d are curved along the length from the blade hub 368 d to the freeend of the blades 369 d.

FIG. 3 shows several embodiments and combinations of embodiments of astacked propeller. Alternatively, a stacked propeller can have differentcharacteristics (e.g., shape, orientation, size) and differentcombination of embodiments to satisfy the design constraints (e.g., loadcapacity, manufacturing limitations) of an aircraft. The stackedpropeller (e.g., starboard stacked propeller 115 a, port stackedpropeller 115 b, front stacked propeller 140 a, rear stacked propeller140 b) can also have a different number of propellers each with adifferent number of blades to improve aircraft efficiency or reducenoise. In one embodiment, a stacked propeller includes a different bladepitch and different twist distributions on each set of blades. A firstpropeller (e.g., a top propeller) may have a lower pitch to induce anairflow, while a second propeller (e.g, a propeller below a toppropeller) can have a higher pitch to accelerate the airflow. The twistdistribution can be configured to stabilize an interaction of a tipvortex (e.g., vortex produced by the tip speed of the upper blade) witha lower blade in order to produce optimal thrust.

FIG. 4A shows a side view of one embodiment of a stacked propeller inone mode of operation and FIG. 4B shows a side view of an embodiment ofa stacked propeller in a different mode of operation. The stackedpropeller shown in FIGS. 4A-4D is substantially similar to the stackedpropeller shown in FIGS. 2A-2B. The schematic includes a first propeller460, a second propeller 462, a blade hub 468, blades 469, a rotor mast470, and an internal cavity 472. FIG. 4A shows a schematic where thefirst propeller 460 and the second propeller 462 are coupled to a rotormast 470. The rotor mast 470 includes an internal cavity 472. In oneembodiment, the rotor mast 470 is a boom (e.g. wing boom 120, tail boom145). In alternative embodiments, the rotor mast 470 can be a nacelle(e.g, nacelle 112). A boom and/or a nacelle can be configured to have asurface profile that matches the blade profile of the first propeller460. This enables a conformal surface fit between the first propeller260 and the rotor mast 470 to minimize drag and flow separation. In oneoperation mode shown by FIG. 4B, the blades 469 of the first propeller460 and the second propeller 462 can be recessed within the internalcavity 472 of the rotor mast 470 in order to reduce drag. The firstpropeller 460 and/or the second propeller 462 can be recessed at onetime in order to cooperate with a mode of operation, described ingreater detail in relation to FIG. 5-11 below.

1.6 Boom Control Effectors

An aircraft can include a boom attached to a region of the aircraft. Inone embodiment, such as illustrated in FIG. 1 , a boom is attached toeach wing of an aircraft 100 and/or the tail of an aircraft 100. Ingeneral, booms contain ancillary items such as fuel tanks. They can alsobe used for providing structural support to an aircraft. In oneembodiment, a boom can include boom control effectors that facilitatedifferent modes of operation of an aircraft.

In the embodiment of FIG. 1 , a propeller (e.g., starboard stackedpropeller 115 a, front stacked propeller 140 a) can be coupled to a boom(e.g., wing boom 120, tail boom 145) to provide lift to an aircraftduring takeoff and landing. Shown in FIG. 1 , a starboard stackedpropeller 115 a is attached to a starboard side wing boom 120 and a portstacked propeller 115 b is attached to a port side wing boom 120. Afront stacked propeller 140 a and a rear stacked propeller 140 b areattached to a tail boom 145. In alternative embodiments, a single rotorpropeller (e.g., a cruise propeller 110) can be attached to a boom.

The booms (e.g., wing boom 120, tail boom 145) can be hollow and can beused to store aircraft components useful for operation. For instance, aboom can include electric motors and batteries to power a propeller(e.g., starboard stacked propeller 115 a, port stacked propeller 115 b)or other aircraft components. In one embodiment, a battery is located atthe bottom of the wing boom 120 and can span the length of the boom 120.In other embodiments, a battery can be located at either end of a wingboom 120 or a tail boom 145 to function as a counterweight to helpmaintain the balance and alignment of aircraft 100. The battery can alsobe placed in a location in the wing boom 120 or the tail boom 145 tominimize aero elastic and whirl flutter resonance during a mode ofoperation. A battery can also be included in another position along theaircraft 100. A battery door can be located on the bottom of the wingboom 120 to allow for removal of the battery powering a propeller (e.g.,starboard stacked propeller 115 a, port stacked propeller 115 b) oranother aircraft component.

In an embodiment where the wing boom 120 and/or the tail boom 145 arehollow, the boom can be used as a resonator to alter the noise signatureof the aircraft 100 during one or more modes of operation. A Helmholtzresonator is a container of gas, such as air, with an open hole. Aresonator can be tuned to the frequency of a propeller such that thenoise resulting from the airflow over a propeller coupled to the boom(e.g. wing boom 120, tail boom 145) is reduced. Sound produced as aresult of pressure fluctuations generated by a propeller can be modifiedby the presence of a tuned volume inside a boom. Tuning the volume canpermit acoustic and aerodynamic modification such that the radiatedsound emitted by a propeller coupled to a boom is reduced. In oneembodiment, a boom (a wing boom 120, a tail boom 145) has an appropriatevolume of air relative to the size of a propeller to act as a resonator.In a mode of operation, when the stacked propellers are deployed (e.g.,takeoff), an internal cavity 472, as described below in relation to FIG.4A, can function as the entrance for airflow into the resonator. Aportion of the air flow over the stacked propeller can flow into theboom (e.g., wing boom 120, tail boom 145) via the internal cavity 472and the frequency can be tuned to reduce the noise produced by thepropeller. In one embodiment, a boom control effector 425 can operate inconjunction with a boom (e.g., wing boom 120, tail boom 145) operatingas a resonator to reduce noise. The rotation frequency, described ingreater detail below, of the boom control effector can be configured totune with the frequency of the resonator such that noise is furthermitigated.

When the aircraft 100 is in a vertical takeoff and landingconfiguration, the propellers (e.g., starboard stacked propeller 115 a,port stacked propeller 115 b) blow air past the wing booms 120 and thetail boom 145 to produce lift. A cross sectional view of an embodimentof a boom (e.g., a tail boom 145, a wing boom 120) is shown by FIGS.4A-4D. FIGS. 4A-4D demonstrate the flow of air over the boom duringdifferent modes of operation. The boom can include a boom controleffector 425 configured to rotate about an axis perpendicular to an axisof rotation 464. A boom control effector can be a single effector asdescribed by FIGS. 4A-4D or a split effector. A split effector mayoperate in conjunction with a boom that operates as a resonator toreduce noise produced by the propeller. The split effector can includetwo boom control effectors attached to a single rotor mast 470.

In one embodiment, a boom can include a rotor mast 470 coupled to a boomcontrol effector 425. A boom control effector 425 can be configured todirect the airflow from a propeller. FIG. 4A illustrates the boomcontrol effector 425 during a mode of operation, such as a verticaltakeoff configuration, as described in greater detail below. The boomcontrol effector 425 is in a neutral position in FIG. 4A. An airflow 490below the propellers (e.g., first propeller 460, second propeller 462)is not separated from the surface of the boom. FIG. 4B illustrates amode of operation, such as a cruise configuration, where the propellers(e.g., first propeller 460, second propeller 462) are recessed withinthe internal cavity 472. When the propellers (e.g., first propeller 460,second propeller 462) are recessed within the cavity 472, the boomcontrol effector 425 may not be in operation (e.g., the boom controleffector remains in a neutral position).

FIGS. 4C-4D illustrate two other modes of operation of a boom controleffector, according to an embodiment. FIGS. 4C-4D show a boom controleffector 425 rotated about an axis perpendicular to an axis of rotation464 (e.g, an axis extending from the page). The angle of the boomcontrol effector 425 directs the downstream airflow 490 in a directionoffset from an axis parallel to the z-axis (i.e. to the left or right)of the boom during various modes of operation, as described in greaterdetail below in relation to FIGS. 5-11 . The angle of the boom controleffector 425 can be manually controlled or automated during differentmodes of operation. The angle can be held constant during a mode ofoperation or may change based on environmental conditions.Alternatively, the boom control effector 425 can be configured tocontinuously oscillate about an axis perpendicular to the axis ofrotation 464. The oscillation frequency can be tuned to align with thefrequency of a boom that functions as a resonator, as described above.In alternative embodiments, the boom control effector 425 can beconfigured to direct the airflow 490 in another direction. The movementof the boom control effector 425 is configured to control the cross windof the propeller and mitigate the acoustic signature of the propeller.The boom control effector 425 can control the direction of the airflow490, which may result in a significant reduction in noise produced bythe propeller. It may also allow for enhanced yaw control of anaircraft. The boom control effector 425 can also improve efficiency andreduce power consumed by the aircraft 100 by realigning the airflow.

In FIGS. 4A-4D, the boom control effector 425 has a teardrop shape. Inother embodiments, the boom control effector 425 can have another shapesuitable for mitigating noise and directing airflow. For instance, theboom control effector 425 can have a split configuration such thatduring a mode of operation, the boom control effector 425 has multiplelongitudinal surfaces that can control airflow direction. The splitconfiguration can be configured to allow the boom to act as a resonator,as described above. In one embodiment, boom control effector 425 and acorresponding boom (e.g., tail boom 145, wing boom 120) have anon-circular cross section to reduce undesired effects (e.g.,aeroelastic and whirl flutter) of aerodynamic forces on the aircraft100. The boom control effector 425 can also have a rectangular endregion coupled to the rotor mast 470 and a pointed or rounded free endregion. The shape of the boom control effector 425 depends on designconsiderations (e.g., size of the propellers, location of thepropellers, aircraft load capacity, etc.) of the aircraft.

A side view of the aircraft 100 including a wing boom and a tail boomare shown in FIG. 4E. The side view illustrates a boom control effector425 coupled to a portion of a wing boom 420. The boom control effector425 extends along the longitudinal surface of the wing boom 420 and ispositioned below a propeller. In one embodiment, the diameter of thepropeller is approximately equal to the length of the boom controleffector 425. In alternative embodiments, the diameter of the propellercan be larger or smaller than the length of the boom control effector425. The internal cavity 472 described above can have a length similarto the length of the boom control effector 425. An aircraft tail boom445 is also shown in FIG. 4E, according to an embodiment. The aircrafttail boom 445 includes a boom control effector 425 that spans the lengthof the tail boom. Two sets of propellers are coupled to the tail boom445. The length of the tail boom control effector 425 is approximatelyequal to the combined diameter of the tail propellers (e.g., frontstacked propeller 140 a, rear stacked propeller 140 b). In alternativeembodiments, the length of the boom control effector 425 can be smalleror larger than the total diameter of the propellers. The internal cavity472 described above can have a length similar to the length of the tailboom control effector 425.

In relation to material composition, boom control effector 425 can bemade from of a single material or can be a composite material able toprovide suitable physical properties for controlling the direction ofairflow behind a propeller. The boom control effector 425 can be madefrom the same material or a different material than the rotor mast 470.The boom control effector 425 may also be exposed to extremeenvironmental conditions, such as wind, rain, hail, and/or extremelyhigh or low temperatures. Thus, the material of the boom controleffector 425 can be compatible with a variety of external conditions.

In relation to mechanical properties, the material of the boom controleffector 425 can have a compressive strength, a shear strength, atensile strength, a strength in bending, an elastic modulus, a hardness,a derivative of the above mechanical properties and/or other propertiesthat enable the boom control effector 425 to direct the airflow 490behind or below a propeller. The boom control effector 425 mayexperience extreme forces during operation including thrust bending,centrifugal and aerodynamic twisting, torque bending and vibrations. Thematerial of the boom control effector 425 can have a strength thatallows the boom control effector 425 to retain its shape under forcesexerted on the boom control effector 425 during various modes ofoperation.

As described above, a boom control effector (e.g., 425) can be includedin a VTOL aircraft 100. A boom control effector 425 can be configured todirect airflow behind or below a propeller included in aircraft 100. Inalternative embodiments, a boom control effector can be included in anyaircraft that includes rotors or propellers, such as a helicopter.

1.7 Modes of Operation

An aircraft mission profile 000 shown in FIGS. 5-11 illustrates theapproximate trajectory of the VTOL aircraft 100 from stage 001-007. Theaircraft and its components shown in FIGS. 5-11 are substantially thesame as the aircraft 100 and the corresponding components shown in FIG.1 (e.g., cruise propellers 510 are substantially the same as cruisepropellers 110). During each stage, components of the aircraft 100 areadjusted such that the center of gravity, center of thrust, andaerodynamic center can be approximately aligned. The components of theaircraft 100 can be adjusted to maximize lift and thrust and reducenoise resulting from airflow over propellers. The adjustable componentsinclude stacked propellers, control surfaces, boom control effectors,and cruise propellers. In alternative embodiments, the aircraft 100 caninclude fewer or more adjustable components for aligning the center ofgravity, center of thrust, and aerodynamic center during stages 001-007of aircraft 100 operation.

FIG. 5 illustrates a taxiing and climb configuration of a VTOL aircraft100, in accordance with an embodiment. Stage 001 corresponds to theparked and taxiing position of the aircraft 100, and stage 002corresponds to the climb (e.g., vertical takeoff) configuration of theaircraft. While the aircraft 100 is parked (e.g., when passengers areentering or exiting the aircraft 100), the stacked propellers (e.g.,front stacked propeller 540 a, rear stacked propeller 540 b) can bestationary, and the wingtip nacelles 512 can be pitched upward such thatthey are perpendicular to the fuselage 535. The aircraft 100 may includeone or more stacked propellers (e.g., starboard stacked propeller 515 a,port stacked propeller 515 b, front stacked propeller 540 a, rearstacked propeller 540 b) located along the aircraft, illustrated by FIG.5 . Each stacked propeller has a first propeller 560 and a secondpropeller 562 that can rotate about a central axis of rotation. Thepropellers (e.g., starboard stacked propeller 515 a, port stackedpropeller 515 b, front stacked propeller 540 a, rear stacked propeller540 b) may also be retracted into a cavity within the aircraft 100 whilethe aircraft is stationary or taxiing. The wing control surfaces 130,tail control surfaces 160, described in relation to FIG. 1 , remain in aneutral position during parking for passenger safety. The rudder 557 canalso remain in a neutral position.

When the aircraft 100 is ready for takeoff, the stacked propellers(e.g., starboard stacked propeller 515 a, port stacked propeller 515 b,front stacked propeller 540 a, rear stacked propeller 540 b) can rotateand increase in rotational speed until the aircraft 100 lifts off theground. During takeoff, stage 002, the nacelles 512 remain at anapproximately 90-degree vertical angle to the fuselage 535 to enable thecruise propellers 510 to provide vertical lift. In one embodiment, theport stacked propeller 515 b, and the rear stacked propeller 540 brotate in a clockwise direction while the starboard stacked propeller515 a, and the front stacked propeller 540 a rotate in acounterclockwise direction during climb.

As the propellers (e.g., starboard stacked propeller 515 a, frontstacked propeller 540 a) rotate, the boom control effectors 525 mayremain in a neutral position. Alternatively, the boom control effectors525 may be angled to yaw the vehicle and guide airflow in a direction tostabilize or otherwise direct the aircraft 100. In most aircrafts, yawmotion is controlled by the rudder 657 of an aircraft. In oneembodiment, the yaw motion is controlled partially or in full by theboom control effectors 525. The yaw motion can also be controlled by a5-10 degree angle of a rudder 557 located on the tail of the aircraft100. Both surfaces may be angled such that the aircraft maintains alevel position during takeoff (e.g., the center of gravity, center ofthrust, and aerodynamic center are approximately aligned). The wingcontrol surfaces 130 can lower 40 degrees and the tail control surfaces160 can lower to approximately 5 to 10 degrees to control aircraftpitch.

FIG. 6 illustrates an early egress transition configuration of a VTOLaircraft 100, in accordance with one or more embodiments. The egresstransition period, stage 003, converts an aircraft from its climb stateto its cruise state. As the aircraft 100 approaches cruising altitude,it begins to transition to a cruise configuration from the verticaltakeoff mode, stage 002. At the beginning of this transition, thenacelles 612 and cruise propellers 610 start to transition downward.Midway through the rotation, the stacked wing propellers (e.g.,starboard stacked propeller 615 a, port stacked propeller 615 b, frontstacked propeller 640 a, rear stacked propeller 640 b) begin to slow butcan remain in the upward position before transitioning to the lateegress mode of operation. The wing control surfaces 130 remain at a 40degree pitch and the tail control surfaces 160 can return to a neutralposition.

FIG. 7A illustrates a late egress transition configuration of a VTOLaircraft 100, in accordance with an embodiment. The aircraft 100approaches the end of the egress transition, stage 003, as the nacelles712 and cruise propellers 710 continue to rotate downward until thenacelles 712 are approximately parallel to the fuselage 735. The stackedpropellers (e.g., starboard stacked propeller 715 a, port stackedpropeller 715 b, front stacked propeller 740 a, rear stacked propeller740 b) can continue to slow their rotation and the first propeller andsecond propeller of each stacked propeller may rotate at the same speed.The wing control surfaces 130 deflect to a neutral position and the tailcontrol surfaces 160 remain in a neutral position. During the earlyegress and late egress transition configuration of a VTOL aircraft 100,the boom control effectors (e.g., 625, 725) and the rudder (e.g., 657,757) can be in a neutral position. Alternatively, the boom controleffectors (e.g., 625, 725) and the rudder (e.g., 657, 757) can be angledto control yaw motion or to reduce noise, particularly in windy orotherwise harsh environmental conditions.

FIG. 7B shows a top view of the blades 769 of a stacked propeller inlate egress transition, according to an embodiment. In FIG. 7B, thefirst propeller 760 is ahead of the second propeller 762 by an azimuthangle 766. As the propeller transitions to cruise, the rotational speedof the propellers (e.g., first propeller 760, second propeller 762) canslow down such that the azimuth angle 766 is zero and the blades 769 arerotating at the same speed, shown in the top view of FIG. 7C. Thepropellers can stop rotating before being retracted into an internalcavity of a rotor mast.

FIG. 8 illustrates a cruise configuration of a VTOL aircraft 100, inaccordance with an embodiment. The cruise configuration, stage 004, isgenerally characterized by a steady, level flight. The wing controlsurfaces 130 and the tail control surfaces 160 remain in a neutralposition. During cruise, the nacelles 812 remain parallel to thefuselage 835, allowing the cruise propellers 810 to propel the aircraft100 at a cruising velocity (e.g., approximately 170 miles per hour). Inone embodiment, the port cruise propeller 810 rotates in a clockwisedirection, and the starboard cruise propeller 810 rotates in acounterclockwise direction. The stacked wing propellers (e.g., starboardstacked propeller 815 a, port stacked propeller 815 b) and stacked tailpropellers (e.g., front stacked propeller 840 a, rear stacked propeller840 b) may be stowed in an internal cavity of a rotor mast, as describedabove in relation to FIGS. 4A-4B, in order to reduce drag. When thepropellers are stored, the aircraft 100 relies on the wings forpropelling forward flight during cruise mode, stage 004. This isbeneficial for the efficiency during forward level flight, becauseaircrafts with single rotors (e.g., helicopters) can be relativelyinefficient during cruise compared to aircrafts with wings. In oneembodiment, 35-40% of the total propeller area, including stackedpropellers and cruise propellers, is active during forward flight. Thismay increase efficiency and avoid rotating or folding of the propellers.Alternatively, the first propeller (e.g., 260) and/or the secondpropeller (e.g., 262) of a stacked propeller (e.g., starboard stackedpropeller 815 a, port stacked propeller 815 b, front stacked propeller840 a, rear stacked propeller 840 b) can remain in its exposed position.

In the embodiment of FIG. 8 , the boom control effectors 825 and therudder 857 remain in a neutral position during a cruise configuration.In particular, the stacked propellers (e.g., port stacked propeller 815b, rear stacked propeller 840 b) may not be rotating or may be recessedwithin a cavity such that a boom (e.g., a wing boom 120, a tail boom145) can function for alternative purposes (e.g., storage). In a secondembodiment, the boom control effectors 825 can be angled to control theairflow behind a propeller. For instance, a boom control effector 825can be attached to a cruise propeller 810. The cruise propeller 810 canbe configured to a boom control effector such that the boom controleffector is the appropriate size and shape for directing an air streamtube behind the cruise propeller 810. During a mode of operation, a boomcontrol effector 825 can direct air flow behind the cruise propeller 810such that the aircraft 100 follows a designated flight path and thenoise produced by the cruise propeller 810 is mitigated. In anembodiment where a boom control effector is attached to a cruisepropeller 810, the air flow behind the propeller may flow in a directionparallel to the fuselage of the aircraft. In this embodiment, or anotherembodiment where the propeller is not a vertical propeller, the boomcontrol effector may be configured to control pitch and/or roll motion.

FIG. 9 illustrates an early ingress transition configuration of a VTOLaircraft 100, in accordance with an embodiment. The early ingresstransition, stage 005, converts the aircraft from a cruise stage 004 toa descent stage 006. As the aircraft 100 begins to transition from thecruise configuration to a vertical descent, the nacelles 912 and cruisepropellers 910 start to transition upwards. The stacked wing propellers(e.g. starboard stacked propeller 915 a, port stacked propeller 915 b)and stacked tail propellers (e.g. front stacked propeller 940 a, rearstacked propeller 940 b) can redeploy from an internal cavity of a rotormast, but may not begin rotating. The wing control surfaces 130 candeflect to a 40 degree angle and the tail control surfaces 160 remain ina neutral position.

FIG. 10 illustrates a late ingress transition configuration of a VTOLaircraft 100, in accordance with an embodiment. The aircraft 100approaches the end of the transition, stage 005, as the nacelles 1012and cruise propellers 1010 fully rotate such that the nacelles 1012 areperpendicular to the fuselage 135. The stacked wing propellers (e.g.starboard stacked propeller 1015 a, port stacked propeller 1015 b) andstacked tail propellers (e.g. front stacked propeller 1040 a, rearstacked propeller 1040 b) begin to rotate and increase in speed. Thefirst propeller and the second propeller of each stacked propeller mayrotate at the same or different speeds. In one embodiment, the stackedpropellers rotate in opposite directions such that the port cruisepropeller 1010, the port stacked propeller 1015 b, and the rear stackedpropeller 1040 b rotate in a clockwise direction while the starboardcruise propeller 1010, the starboard stacked propeller 1015 a, and thefront stacked propeller 1040 a rotate in a counterclockwise direction.The wing control surfaces 130 are pitched down to 40 degrees and thetail control surfaces 160 remain in a neutral position.

During the early ingress and late ingress transition of the aircraft(FIGS. 9-10 ), the boom control effectors (e.g. 925, 1025) may remain ina neutral position while the propellers are not rotating. In otherembodiments, the boom control effectors (e.g. 925, 1025) can be tiltedto control yaw movement and/or reduce noise if the propellers beginrotating. In one embodiment, the boom control effectors (e.g. 925, 1025)may have the same angle with respect to the axis of rotation. In otherembodiments, the boom control effectors (e.g. 925, 1025) may havedifferent angles for guiding the airflow from the propellers withrespect to the axis of rotation for each propeller. The rudder (e.g.,957, 1057) attached to the tail of the aircraft can also remain in aneutral position during the ingress transition period.

FIG. 11 illustrates a descent configuration of a VTOL aircraft 100, inaccordance with an embodiment. The descent stage 006 converts theaircraft from the ingress transition, stage 005, to a landing stage 007.As the aircraft 100 descends toward a landing area, the cruisepropellers 1110, and the stacked propellers (e.g. starboard stackedpropeller 1115 a, port stacked propeller 915 b, front stacked propeller1140 a, rear stacked propeller 1140 b) rotate to generate lift. Thestacked propellers function to provide lift for vertical landing andbalance the aircraft during landing. The propellers decrease inrotational speed as the aircraft 100 touches down. The boom controleffectors 1125 and the rudder 1157 may be titled for yaw control andnoise control. The wing control surfaces 130 are pitched down to 40degrees and the tail control surfaces 160 can lower to approximately 5to 10 degrees to control pitch. After the aircraft 100 touches down, itreturns to the parked configuration such that the propellers (e.g.starboard stacked propeller 1115 a, port stacked propeller 1115 b, frontstacked propeller 1140 a, rear stacked propeller 1140 b) stop rotating.The boom control effectors 1125, the wing control surfaces 130, and thetail control surfaces 160 return to a neutral position.

The description of a stacked propeller used by the entities of FIGS.5-11 can vary depending upon the embodiment and the requirements of theaircraft system. For example, the aircraft might include stackedpropellers located along the fuselage or other areas of the aircraft.The aircraft may include more or less stacked propellers than thoseshown in FIGS. 5-11 . The stacked propellers and/or aircraft may lacksome elements included in the above description. The operation of thestacked propellers is not limited to the description of FIGS. 5-11 . Forexample, the boom control effectors may be tilted or neutral in modes ofoperation not described above, depending on aircraft or environmentalconditions.

ADDITIONAL CONSIDERATIONS

The description has been presented for the purpose of illustration; itis not intended to be exhaustive or to limit the invention to theprecise forms disclosed. Persons skilled in the relevant art canappreciate that many modifications and variations are possible in lightof the above disclosure.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the invention be limited not bythis detailed description but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsof the invention is intended to be illustrative but not limiting of thescope of the invention.

The invention claimed is:
 1. An aircraft comprising: a fuselage; anM-wing coupled to the fuselage, the M-wing having a port portion and astarboard portion, the port portion and the starboard portion eachincluding a first segment extending outwardly from the fuselage to arespective inflection point and a second segment extending outwardlyfrom each respective inflection point, the first segment and the secondsegment joining at a non-zero angle at the inflection point; a tailregion extending from the fuselage; a set of propellers including afirst propeller and a second propeller, the first propeller is coupledto the port portion of the M-wing and the second propeller is coupled tothe starboard portion of the M-wing; a first wing boom coupled to theport portion of the M-wing proximate the inflection point of the portportion, the first propeller being coupled behind the M-wing to thefirst wing boom; and a second wing boom coupled to the starboard portionof the M-wing proximate the inflection point of the starboard portion,the second propeller being coupled behind the M-wing to the second wingboom.
 2. The aircraft of claim 1, wherein the first propeller and thesecond propeller are located in front of a center of gravity of theaircraft.
 3. The aircraft of claim 1, wherein the first wing boom or thesecond wing boom includes a boom control effector configured to directairflow generated by the first propeller or second propeller.
 4. Theaircraft of claim 1, wherein the first wing boom or the second wing boomis hollow and is configured as a resonator tuned to a frequency of thefirst propeller or second propeller during a mode of operation.
 5. Theaircraft of claim 1, wherein the first propeller and the secondpropeller each include a set of co-rotating propellers.
 6. The aircraftof claim 5, further comprising: a first motor coupled to a firstco-rotating propeller of each set, and a second motor coupled to asecond co-rotating propeller of each set.
 7. The aircraft of claim 6,wherein one of the first motor or the second motor controls the firstand second co-rotating propellers when the other motor fails.
 8. Theaircraft of claim 1, further comprising; a first cruise propellercoupled to a first nacelle at a free end of the starboard portion of theM-wing; and a second cruise propeller coupled to a second nacelle at afree end of the port portion of the M-wing.
 9. The aircraft of claim 8,wherein the first nacelle and the second nacelle are configured to beperpendicular to the fuselage during a first mode of operation andparallel to the fuselage during a second mode of operation.
 10. Theaircraft of claim 8, wherein the first cruise propeller rotates in adirection opposite to a rotation of the second cruise propeller during amode of operation.
 11. The aircraft of claim 1, wherein the tail regionincludes a T-tail having a fin with a rudder configured to control yawmotion of the aircraft and a tail plane coupled perpendicularly to thefin.
 12. The aircraft of claim 11, wherein the tail plane includes atail control surface configured to rotate about an axis parallel to thetail plane to control the pitch of the aircraft.
 13. The aircraft ofclaim 1, further comprising an internal cavity located in the first wingboom and an internal cavity located in the second wing boom, eachinternal cavity being configured to contain one of the first propelleror the second propeller during a mode of operation.
 14. The aircraft ofclaim 1, wherein the first propeller rotates in a direction opposite toa rotational direction of the second propeller during a mode ofoperation.
 15. The aircraft of claim 1, wherein the tail region includesa tail boom.
 16. The aircraft of claim 15, wherein the tail boomincludes a boom control effector configured to direct airflow generatedby a propeller.
 17. The aircraft of claim 15, wherein the tail boom ishollow and is configured as a resonator tuned to a frequency of apropeller during a mode of operation.
 18. The aircraft of claim 1,further comprising a third propeller coupled to the tail region.
 19. Theaircraft of claim 18, wherein the third propeller includes a set ofco-rotating propellers.
 20. The aircraft of claim 18, furthercomprising: a fourth propeller coupled to the tail region of theaircraft, wherein the fourth propeller rotates in a direction oppositeto a rotational direction of the third propeller during a mode ofoperation.